MIGRATORY birds use the Earth's magnetic fields for refuelling as well as navigation, a new study showed yesterday.

MIGRATORY birds use the Earth's magnetic fields for refuelling as well as navigation, a new study showed yesterday.

Scientists found that thrush nightingales subjected to a magnetic field simulating the one in northern Egypt began stocking up on food.

In the wild, the birds eat voraciously on their last stop-over before crossing the Sahara desert - a distance of 1,500km.

The new findings, reported by Swedish researchers in the journal Nature, suggest that magnetism is the trigger which persuades the birds to start putting on weight.

Scientists already had evidence that migratory birds use the Earth's magnetic field as a navigational aid, along with celestial cues.

The loggerhead turtle is also known to navigate its way around feeding grounds in the north Atlantic using a form of internal magnetic compass.

But what prompted the refuelling stops of migratory birds was a mystery. Seasonal changes alone could not be the answer, because migrations started at various times and weather and feeding conditions were liable to change.

Thord Fransson, from Stockholm University, and colleagues, caught thrush nightingales in Sweden just before their first migration.

Half the birds were subjected to a magnetic field, which was gradually tuned to match that of northern Egypt. Other birds experienced the ambient magnetic field found in Sweden.

Birds subjected to the changing magnetic field began to eat more once the field became the same as that in Egypt.

These birds increased in mass by

3.5g from days six to 11 of the experiment. The "control" birds, on the other hand, only put on 1.1g.

The researchers wrote in Nature, "We have discovered a surprising external cue that helps to optimise this bird's chances of successful migration, and which works in concordance with orientation behaviour and endogenous rhythm to provide precise information about geographical position when such information is crucial.

"We cannot say yet say whether the fat-deposition response described here is an evolved response to the magnetic field in a specific area, or whether the birds are reacting to the latitudinal change in magnetic field."